In the fabrication of precision optical components (telescope mirrors, lenses for imaging systems, laser windows, etc.), achievement of surface form and finish is critical. To rough out the basic optical form, any number of processes may be employed including diamond machining, abrasive grinding, or even molding. Such processes, however, produce only imprecise surface form and leave a rough surface finish. Even extremely fine grinding, using grit sizes in the few micron range, yields a microscopically pitted and frosty surface. Precision optical fabrication, therefore, requires a finishing step to achieve near molecular level smoothness and extreme accuracy in final surface figure. This final operation, commonly referred to as polishing, is generally critical within the field of precision optical fabrication.
Within the last few decades, tremendous advances have been made in the general art of surface polishing. Techniques such as chemical mechanical planarization (CMP) are now available for economical achievement of nanometer scale smoothness in semiconductor manufacturing. For optical polishing, magnetorheological finishing has proven value in specific precision finishing applications. For the manufacture of most precision optical components, however, traditional lap polishing remains the most economical and viable approach.
Remarkably, Sir Isaac Newton is credited with inventing the lap polishing method, which in essential respects is the basis for most modern precision optical finishing. Newton found that although fine grinding leaves optical surfaces microscopically pitted, an incredibly smooth finish might be obtained on glass (and like surfaces) if worked against a pitch material in the presence of a slurry containing soft abrasive particles. In the roughly 300 years since Newton originally published this technique, the basic methodology has become a staple optical finishing technique.
In pitch lapping, a hard support is typically coated with a layer of natural pitch, comprised of pine rosin or petroleum based resin, to form a lap of cut or molded pitch facets. Often, the pitch is melted, cast directly onto the support, cut into facets or, in some other fashion, molded into facets attached to the rigid support. Pitch, formulated for this application, displays slow creep or flow under the action of stress (although it outwardly appears as a hard resinous solid much like amber or hard sugar candy). Pressed against an optical workpiece, pitch facets slowly conform to provide an intimate mate with the surface. Subsequent working against the lap, in the presence of a particle slurry leads to charging of the lap surface with embedded particles. As a result, particles entrapped at the pitch surface are delicately dragged over the work and, through complex mechanisms, smooth the surface to produce an extremely high polish. Beyond polishing, lapping also delicately removes material from the optical surface and, thus, enables extremely fine adjustment of surface figure at the final stages of finishing.
Although the underlying mechanisms of pitch polishing remain a subject for investigation, pitch flow or creep is accepted as an important enabling phenomenon. Apparently, flow of the lap to mate with the optical surface is needed for the achievement of both uniform polish, and control of surface figure. Certainly, it is widely accepted that materials, which do not flow and conform to the optical work must either be preconditioned to provide conformance, or produce poor polishing results. Waxes, for example, produce a polishing effect but often with characteristic non-uniform lemon peel texture. Materials such as TEFLON fluorocarbon polymers may also be used for lapping but only following exhaustive diamond conditioning to provide close mating. Flow in pitch, thus, offers a practical means to achieve intimate microscopic lap mating and consequent polishing uniformity/control.
Due to the importance of flow in lap performance, optical polishing pitches are commonly formulated to possess levels of flow appropriate for a given application. The so called hardness of a pitch, or its resistance to flow in response to stress, is determined by mixing resins with different characteristics or plasticizing a given material with additives. Generally, pine rosin, plasticized with turpentine or other similar materials, has long been used as a polishing pitch. For example, one published account of a pitch formulation containing pine rosin dates back to the 1940s.
Currently, several brands of optical polishing pitch, blended to achieve different levels of hardness, are available commercially. Swiss manufactured GUGOLZ pitches, based on proprietary pine derivatives, are used extensively within the field of precision optics. Universal Photonics, Inc. of Hicksville, N.Y., also offers similar polishing pitches formulated across a very broad range of hardness. Beyond pine derivatives, Cycad Products, of Las Vegas, N. Mex., offers a range of pitches based on proprietary petroleum refining residuals and these pitches also find specific applications in optical finishing.
While the use of pine and petroleum resins as a base for optical polishing pitches has a long history, associated technology has specific limitations, which are addressed by the current invention.
Pine based pitches are typically derived from gum pine rosin, or similar related materials which, chemically, are largely comprised of materials refined from the sap of pine trees. While these materials are dominantly comprised of resin acids (such as abietic acid), exact weight distribution among these acids vary and many residual compounds are present depending on pine species, harvest location, and growing conditions. Consequently, the exact chemical makeup of pitches thus derived is difficult to control and, inherently, comprises an extremely complex blend of compounds, which are difficult to reproduce consistently over time. Similarly, petroleum based pitches, derived from refining residuals, contain a complex blend of isomers, oligimers, cyclic anthracitic compounds, and the like, depending on exact crude source.
While in principle it is possible to manufacture consistent optical pitches derived from either pine resin or petroleum pitch, chemical complexity and inconsistency in raw natural constituents present significant practical barriers the achievement of precise physical properties. Pine derivatives from a given source, such as Hercules, Inc. in Brunswick, Ga., for example, contain variable fractions of low molecular weight compounds, which, like turpentine added to deliberately induce creep characteristics, induce varying levels of hardness. Since even a fraction of one percent (by weight) of low molecular weight constituents changes flow characteristics dramatically, precise blending with additives to achieve precise pitch hardness is hampered by associated variability. Similarly, pitches derived from petroleum residuals contain small fractions of low molecular weight compounds, which vary in concentration and composition, leading to analogous difficulty in precision control of hardness.
Variability in the physical properties of natural pitches, particularly hardness, is a serious issue in the manufacture of precision optics. Since reproducibility in polishing operations depends upon the properties of the pitch employed, lot-to-lot variability in pitch characteristics translates into costly and unwanted process troubleshooting.
Notwithstanding ongoing efforts to characterize and track the lot-to-lot properties of existing natural pitches, variability is a major current issue for most manufacturers of precision optics. While quality control measurements of natural pitch characteristics may be employed to select and screen lots of material for a given finishing operation, such screening is time consuming and generally costly. Consequently, lower variability alternatives would represent a fundamental advancement in the art of optical polishing.
Beyond the issue of chemical purity and consistency in manufacture, existing pine and petroleum pitches are relatively unstable both during melt molding to produce laps and in use. Since the base resins typically comprise low molecular weight components, melting or long-term exposure to air can lead to significant changes in hardness due to loss of low boiling volatiles. For this reason, opticians must take great care in the melting of pitch (to fabricate laps) such that exposure to high temperature and drying/hardening of the material is limited. In addition, laps constructed of such pitches have a limited life in part due to drying of volatiles from the material and consequent hardening.
It is also important to recognize that many existing natural pitch formulations, particularly those comprising pine derivatives, are subject to reactions with oxygen over time. Most pine derivatives, for example, become rancid much like unsaturated fats or cooking oils, within a few weeks on exposure to air due to oxidative reactions. Although formulation with additives may inhibit such reactions to some extent, oxidation is another inherent instability associated with natural resins.
In large part due to the above issues, current practitioners in the art of pitch polishing face many practical complexities and must, in general, develop considerable formulation and processing expertise to achieve success. Blending of different commercial pitches is often required to overcome variability in hardness and or polishing performance. All too often, trial and error blending to modify hardness, enhance surface wetting, or insure proper charging with polishing agents, is necessary. In addition, extreme care is required in molding of natural pitches to prevent, degradation or drying of the material during melt processing, requires considerable care and experience. While many opticians have developed remarkable skill and intuition in the general art of pitch manipulation, more stable and pure materials, having precise and controlled properties, would enable more systematic, and less costly, optical process engineering. In general, the ability to scientifically formulate exact hardness targets, utilizing materials having known chemistry in combination with additives yielding well-characterized effects, would be of great value in many optical polishing applications.
What is needed, therefore, are pitch materials comprising chemically pure substances, which may be precisely formulated to yield precision control of physical properties including pitch hardness. In addition, such materials are needed which do not contain significant volatile content and, thus, are not susceptible to drying over time or during melt processing. Finally, optical polishing pitches, which are impervious to oxidation on exposure to air, are generally needed in the art.